Adjustable shock absorbers for motor vehicles having electromagnetically displaceable throttling members are known from West German patent specification No. 10 84 528, West German published patent application No. 32 15 614 and West German published patent application No. 32 41 984. In these shock absorbers a throttling plate mounted to be rotationally displaceable in the damping piston, is used as a throttling member.
The throttling plate is provided with appropriately positioned holes therethrough which can be brought into alignment to a greater or lesser degree with through-flow passages provided in the piston body. Alternatively a through-flow hole lying in the bypass to the fluid flow passages of the piston is freed to a greater or lesser extent.
The displacement of the throttling plate is effected by means of a control rod which is connected rigidly to the plate and which extends through the full length of the piston rod, which is hollow. The control rod can be brought into different rotational set positions by means of an electromagnet mounted at the free end of the piston rod in a suitably projecting piston rod head.
In another known adjustable shock absorber for motor vehicles, as described in West German published patent application No. 29 11 768, an electromagnet coil is mounted at the outer end of a hollow piston rod, and a control rod which serves as an armature for the coil extends through the full length of the piston rod and is axially displaceable therein. The inner end of the control rod permits bypass openings in the piston rod in the vicinity of the damping piston to be closed off to a greater or lesser degree.
In these known electromagnetically adjustable shock absorbers it is a common feature that in spite of far-reaching objectives it is only possible to achieve a coarse presetting of the throttling member and thus also of the desired variable damping force behavior, since, in respect of their setting means, these shock absorbers are very sluggish in terms of their inertia. The fact that the control rod extends through the hollow piston rod to its full length and lies between the electromagnetic drive means on the one hand and the throttling member on the other hand contributes to this to a substantial degree.
Electromagnetically adjustable shock absorbers of the type referred to above are also already known from French patent specifications Nos. 1094025, 1095506 and 1130621. In these shock absorbers electromagnetic drive means comprising magnetic circuits and coil windings and acting upon the throttling member are mounted in the damping piston immediately adjacent to the fluid flow passages and the throttling member. The throttling member is, in all these cases, formed as a throttling piston which is mounted to be displaceable in the longitudinal direction of the piston rod against a spring bias. The throttling piston is set to a fully defined throttling position in dependence upon the particular current flow in the coil winding and is maintained in that position, and indeed independently of the changing fluid pressures arising in the working chamber compartments, since those pressures are compensated by the equal size fluid pressure reaction surfaces provided on the throttling piston, whereby the changing pressures have no effect on the position of the throttling piston. Consequently, it is still only possible to achieve a comparatively coarse damping setting, with throttling holes of constant cross-section producing a characteristic exponential damping force curve in dependence upon the piston speed.